1. Field of the Invention
This invention relates to the fluidized catalytic cracking (FCC) conversion of heavy hydrocarbons into lighter hydrocarbons with a fluidized stream of catalyst particles. More specifically, this invention relates to the process and apparatus for stripping catalyst from the FCC reaction process.
2. Description of the Prior Art
Catalytic cracking is accomplished by contacting hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. A high temperature regeneration within a regeneration zone operation burns coke from the catalyst. Coke-containing catalyst, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known by those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively.
A majority of the hydrocarbon vapors that contact the catalyst in the reaction zone are separated from the solid particles by ballistic and/or centrifugal separation methods within the reaction zone. However, the catalyst particles employed in an FCC process have a large surface area, which is due to a great multitude of pores located in the particles. As a result, the catalytic materials retain hydrocarbons within their pores and upon the external surface of the catalyst. Although the quantity of hydrocarbons retained on each individual catalyst particle is very small, the large amount of catalyst and the high catalyst circulation rate which is typically used in a modern FCC process results in a significant quantity of hydrocarbons being withdrawn from the reaction zone with the catalyst.
Therefore, it is common practice to remove, or strip, hydrocarbons from spent catalyst prior to passing it into the regeneration zone. It is important to remove retained spent hydrocarbons from the spent catalyst for process and economic reasons. First, hydrocarbons that entered the regenerator increase its carbon-burning load and can result in excessive regenerator temperatures. Stripping hydrocarbons from the catalyst also allows recovery of the hydrocarbons as products. Avoiding the unnecessary burning of hydrocarbons is especially important during the processing of heavy (relatively high molecular weight) feedstocks, since processing these feedstocks increases the deposition of coke on the catalyst during the reaction (in comparison to the coking rate with light feedstocks) and raises the combustion load in the regeneration zone. Higher combustion loads lead to higher temperatures which at some point may damage the catalyst or exceed the metallurgical design limits of the regeneration apparatus.
The most common method of stripping the catalyst passes a stripping gas, usually steam, through a flowing stream of catalyst, countercurrent to its direction of flow. Such steam stripping operations, with varying degrees of efficiency, remove the hydrocarbon vapors which are entrained with the catalyst and hydrocarbons which are adsorbed on the catalyst. Contact of the catalyst with a stripping medium may be accomplished in a simple open vessel as demonstrated by U.S. Pat. No. 4,481,103.
The efficiency of catalyst stripping is increased by using vertically spaced baffles to cascade the catalyst from side to side as it moves down a stripping apparatus and countercurrently contacts a stripping medium. Moving the catalyst horizontally increases contact between the catalyst and the stripping medium so that more hydrocarbons are removed from the catalyst. In these arrangements, the catalyst is given a labyrinthine path through a series of baffles located at different levels. Catalyst and gas contact is increased by this arrangement that leaves no open vertical path of significant cross-section through the stripping apparatus. Further examples of these stripping devices for FCC units are shown in U.S. Pat. Nos. 2,440,620; 2,612,438; 3,894,932; 4,414,100; and 4,364,905. These references show the typical stripper arrangement having a stripper vessel, a series of baffles in the form of frustoconical sections that direct the catalyst inwardly onto a baffle in a series of centrally located conical or frusto conical baffles that divert the catalyst outwardly onto the outer baffles. The stripping medium enters from below the lower baffle in the series and continues rising upward from the bottom of one baffle to the bottom of the next succeeding baffle. Variations in the baffles include the addition of skirts about the trailing edge of the baffle as depicted in U.S. Pat. No. 2,994,659 and the use of multiple linear baffle sections at different baffle levels as demonstrated in FIG. 3 of U.S. Pat. No. 4,500,423. A variation in introducing the stripping medium is shown in U.S. Pat. No. 2,541,801 where a quantity of fluidizing gas is admitted at a number of discrete locations.
The use of a stripping vessel subadjacent to a reactor vessel in combination with a separate larger stripper vessel is known from U.S. Pat. No. 4,481,103.
The use of a small stripping vessel within a reactor vessel is known from U.S. Pat. No. 2,838,382.
In order to achieve good stripping of the catalyst and the increased product yield and enhanced regenerator operation associated therewith, relatively large amounts of stripping medium have been required. For the most common stripping medium, steam, the average requirement throughout the industry is well above 1.5 kg of steam per 1000 kg of catalyst for thorough catalyst stripping. The costs associated with this addition of stripping medium are significant. In the case of steam, the costs include capital expenses and utility expenses associated with supplying the steam and removing the resulting water via downstream separation facilities. Any reduction in the amount of steam required to achieve good catalyst stripping will yield substantial economic benefits to the FCC process. As a result, it is an objective of any new stripping design to minimize the addition of stripping medium while maintaining the benefits of good catalyst stripping throughout the FCC process unit.
Process configurations for FCC units have undergone considerable change since the introduction of such process units in the1940's. One well known configuration of FCC unit that gained wide acceptance during the 1950's and 1960's is a stacked FCC reactor and regenerator. This design comprises a reactor vessel stacked one on top of a regenerator vessel. Regenerated catalyst flows from the regeneration vessel through a regenerator standpipe into a riser where it contacts an FCC charge stock. Expanding gases from the charge stock and fluidizing medium convey the catalyst up an external riser and into the reactor vessel. Cyclone separators in the reactor divide the catalyst from reacted feed vapors which pass into an upper recovery line while the catalyst collects in the bottom of the reactor. A stripping vessel, supported from the side of the reactor vessel, receives spent catalyst from the reaction zone. Steam rises from the bottom of the stripper, countercurrent to the downward flow of catalyst, and removes sorbed hydrocarbons from the catalyst. Spent catalyst continues its downward movement from the stripper vessel through a reactor standpipe and into a dense fluidized catalyst bed contained within the regeneration vessel. Coke on the spent catalyst reacts with oxygen in an air stream that ascends through the regeneration vessel and ultimately becomes regeneration gas. Again, cyclone separators at the top of the regenerator return catalyst particles to the dense bed and deliver a relatively catalyst-free regeneration gas to an overhead gas conduit.
Changes in the FCC equipment and the operation of FCC units have decreased the utility of older FCC reactors especially stacked reactor regenerator designs. Two such changes include the adoption of all riser cracking in FCC units and the use of higher efficiency cyclones. In early FCC processes, after initial contact with the catalyst and oil in a relatively small diameter riser conduit, reaction of the catalyst and oil feed continued in a dense bed contained within the reactor vessel. Modern FCC units have extended the reactor riser and eliminated the dense bed in the reaction zone so that the majority of any cracking reactions occur in the riser conduit. The use of cyclones to centrifically separate gases from catalyst particles is the principal method for separating catalyst and hydrocarbons in the FCC reactor. In an effort to decrease particulate emissions from FCC units, higher efficiencies are sought from the cyclones. Higher efficiencies generally require a longer cyclone length in order to allow a longer vortex formation within the cyclone and obtain a more complete separation of the catalyst particles from the gases.
Both all riser cracking and the use of more efficient cyclones having longer lengths pose special problems for reactors having short tangent lengths and especially stacked FCC reactors having side strippers mounted thereon. In order to fit the longer cyclones within the reactor vessel, a greater length is needed than is sometimes provided in older reactor vessels. The addition of riser cracking has compounded the problem in some cases by the elimination of the dense bed of fluidized catalyst. In order to function properly, the lower outlet or discharge leg of the cyclone must empty into or above a fluidized bed of catalyst. Before all riser cracking all of the gases and catalyst from the riser entered the lower portion of the reactor to fluidize all of the catalyst in the lower portion of the reactor vessel. Thus, a dense fluidized bed was maintained through the entire lower portion of the reactor. All riser cracking adds the catalyst and gases at an upper elevation of the reactor vessel so that catalyst, located below the point where catalyst is withdrawn from the reactor, forms a stagnant bed. Where this catalyst withdrawal point is located at a relatively high location on the vessel shell, it significantly raises the lowermost point at which the cyclone discharge legs can be located, thereby limiting the total length available for the cyclones.
In the case of stacked FCC units having side mounted strippers, the catalyst withdrawal point is on the side of the reactor vessel some distance up from the bottom of the reactor. Conversion of these units to all riser cracking and the elimination of the dense fluidized bed thereby formed a stagnant layer of catalyst that sloped upward from the stripper outlet to the opposite side of the reactor vessel. This upwardly sloping layer of stagnant catalyst severely restricted the internal length of the reactor vessel that was available for the reactor cyclones. One solution to the length restrictions imposed by the stagnant layer of catalyst is to convert the stagnant layer of catalyst to a fluidized bed by the addition of a fluidizing medium to the bottom of the reactor vessel. However, this is not practical due to the large size of the reactor vessel and the volume of fluidizing medium required for fluidization. In addition, there's a limited amount of clearance available between the bottom of the reactor and the top of the regenerator which prevents the use of a typical stripper outlet for the withdrawal of catalyst from the bottom of the reactor.
In view of the large number of older style FCC units that are still in existence, it would be highly useful to have a method and arrangement for stripping FCC catalyst that would overcome the problems associated with all riser cracking and the use of more efficient cyclones.
More recently efforts to improve all riser cracking operation seek to reduce the post riser residence time of reactants. Reductions in post riser residence time apply to overall residence time in the reactor and stripping vessel and post riser contact time of hydrocarbons with catalyst. Post riser residence time reductions improve product yields and selectivity. Conventional stripping arrangements operate with a high downward superficial catalyst flux that increase the contact time of entrained hydrocarbons with the catalyst. It would also be useful to have a stripping arrangement that reduces the post riser residence time and catalyst contact time.